(CNN) -- Casualties could have been reduced by half among Marines in Iraq if specially armored vehicles had been deployed more quickly in some cases, a report to the Pentagon says.

MRAP vehicles are started up and inspected in South Carolina in January before being shipped to Iraq.

Marine Corps spokesman Col. David Lapan said the Defense Department's inspector general wants to investigate the report's claims that bureaucratic delays undermined the program to develop the armored vehicles.

The program was designed to provide combat forces with Mine Resistant Ambush Protected Vehicles, known by the acronym MRAPs.

"If the mass procurement and fielding of MRAPs had begun in 2005 in response to the known and acknowledged threats at that time, as the USMC is doing today, hundreds of deaths and injuries could have been prevented," Gayl wrote in the report.

The report concluded that bureaucratic delays plagued the program at the height of the insurgency, when U.S. troops were regularly being attacked and killed by roadside bombs. Watch an MRAP roll with gun blazing »

Until MRAPs began arriving in Iraq in large numbers in 2007, troops had limited protection in armored Humvees.

Mine-resistant vehicles

The U.S. military has contracts with several manufacturers for mine-resistant, ambush-protected vehicles, or MRAPs. The vehicles come in three categories, all of which feature a V-shaped hull to deflect the force of an explosion.

Category I- Transports four to six personnel- Designed for operations in confined spaces- Missions include mounted patrols, reconnaissance and communication

Category II- Longer version of Category I- Carries up to 10- Missions include leading convoys, troop transport and ambulance

The MRAPs, which are the newest armored vehicles, have a V-shaped hull that helps deflect the blast of a roadside bomb.

Gayl wrote the January 22 report about the MRAP program, but it is not considered an official document.

"In the past, Gayl has leveled serious charges about the wartime acquisition process; therefore his supervisors provided him an opportunity, on government time, to conduct a series of case studies to attempt to validate his arguments," the Marine Corps said in e-mail response to a CNN query.

"If Gayl was able to provide compelling evidence of flaws or errors in the system, his supervisors would then determine how best to address his concerns with senior Marine Corps leaders in order to effect positive change."

In the same e-mail response, the Marine Corps explained that several improvements to acquisition have been made, and officials note that Defense Secretary Robert Gates has made MRAP a top priority.

"However, because of the seriousness of the allegations," the Marine Corps decided to ask for the investigation.

Don't Miss

In northern Iraq on Tuesday, a bomber detonated a suicide vest, killing nine Iraqis, U.S. and Iraqi officials said. The incident took place north of Tal Afar in Nineveh province.

Police said several al Qaeda in Iraq members were aboard the bus, and one of them detonated the device after arguing with the driver before the bus reached a checkpoint.

An earlier statement from the U.S. military said the bomber detonated his vest outside the bus after the vehicle was stopped at the checkpoint.

No security forces were among the casualties, the U.S. military said.

In central Baghdad, an Iraqi soldier was killed and four people were wounded Tuesday when a roadside bomb struck an army convoy, the Interior Ministry said. Two of the wounded were soldiers.

U.S. troops in northern Iraq killed seven insurgents on Monday in a firefight, the military said Tuesday.

The Americans were ambushed east of Khan Bani Saad in Diyala province during a raid to capture an al Qaeda in Iraq operative, the Pentagon said.

"When the soldiers closed within 30 feet of the target house, they came under attack by small arms fire and grenades" and three U.S. troops were wounded, the military said.

"The troops, shielding the wounded with their bodies, returned fire, killing three terrorists. A scout weapons team observed several more fighters fleeing the target house and engaged them with rockets, killing four."

The targeted militant and another were detained, and the wounded troops were in stable condition, the military said.E-mail to a friend